1. Field of the Invention
The present invention relates to semiconductor devices, more specifically to semiconductors with MOS transistors having; source and drain regions of the second conductivity type which are spaced at intervals from each other and formed on a semiconductor layer of the first conductivity type; a channel region of the first conductivity type which is formed between the source and drain regions; and a gate electrode formed on the channel region with a gate insulator film in between the gate electrode and the channel region.
2. Description of Related Art
Transistors which are termed “driver transistors” are known as MOS transistors. In MOS transistors, transistors with a large channel width (W) and a large output current are generally termed “driver transistors”.
FIG. 8 shows a schematic plan view and a cross-sectional view illustrating the driver transistors according to the related art. The cross-sectional view is along the position X-X of the plan view.
A LOCOS (Local Oxidation of Silicon) oxide film 103 is formed in the surface of a p-type silicon substrate 101 in order to define a position for the driver transistor to be formed. An n-type well 105 is formed in the position of the p-type silicon substrate 101. A plurality of source regions 107s and drain regions 107d composed of p-type impurity-diffused layers are provided in the n-type well 105. The source region 107s and the drain region 107d are spaced from each other and arranged in alternate shifts.
A gate electrode 111 made of poly-silicon is formed on the n-type well 105 between the source region 107s and the drain region 107d with a gate insulator film 109 in between the gate electrode 111 and the n-type well 105. The gate electrode 111 is formed in a position between the source region 107s and the drain region 107d. FIG. 8 shows a transistor with four gate electrodes, but it is common for dozens of gate electrodes to be provided in order to design a large channel width.
A contact region for a substrate potential 107b (hereinafter “contact region for substrate potential” is referred to as “contact region”) made of an n-type impurity-diffused layer is provided in the n-type well 105 in such a way as to enclose the positions where the source region 107s and drain region 107d are formed. The contact region 107b is one for obtaining the substrate potential, namely an electrical potential of the n-type well 105.
An interlayer dielectric film (not shown in the figures) is formed all over the silicon substrate 1 and contains a region where the source region 107s, the drain region 107d, the gate electrode 111 and the contact region 107b are formed. A contact 113s is formed on the source region 107s. A contact 113d is formed on the drain region 107d. A contact 1136 is formed on the contact region 107b. A contact is formed on the gate electrode 111 in a region which is not shown in the figures. These contacts are made of a conducting material.
A plurality of the source regions 107s is electrically supplied and all set to be the same predetermined electric potential. A plurality of the drain regions 107d is electrically supplied and all set to be the same predetermined electric potential.
A plurality of the gate electrodes 111 is electrically supplied and all set to be the same predetermined electric potential. The contact region 107b is electrically supplied and set to be an electric potential that is the same as that of the source region 107s or the drain region 107d. 
Or the contact region 1076 is electrically supplied and set to be another electric potential different from that of the source region 107s and that of the drain region 107d. 
As shown in FIG. 8, it is characteristic of the driver transistor that the source region 107s and the drain region 107d are arranged in alternate shifts on both sides of the gate electrodes 111.
Once the driver transistor has been “on” (a state capable of carrying an electric current), an electric current is carried in a direction of the arrow as shown in the cross-sectional view of FIG. 8.
In summary, one source region 107s and one drain region 107d act on the bilateral gate electrodes 111,111, so that it is possible to accomplish a layout that can carry a large current in a small area.
Here, a role of the contact region 107b will be described. The contact region 107b is arranged for providing a predetermined electric potential to the n-type well 105. In this related art, a case is explained of providing ground potential (electric potential at 0V) to the contact region 107b and the n-type well 105. If the ground potential is provided to the contact region 1076, all of the contact region 107b and the n-type well 105 should be the ground potential in theory. However, a phenomenon as explained below occurs in real driver transistors.
As explained previously, the driver transistors are sometimes designed with a remarkably large channel width, for example, over 100,000 micrometers due to a need to carry a large current.
In this case, not only the channel width (vertical direction in FIG. 8) but also the channel length (lateral direction in FIG. 8) is enlarged. As a result, a layout range of the driver transistor becomes very large.
If the layout range of the driver transistor is enlarged, it is known that the substrate potential becomes out of the ideal state at a position away from the contact region 107b due to an influence of the substrate potential by impact ionization, which mainly stems from a large resistance of the n-type well 105.
FIG. 9 shows a schematic plan view and a cross-sectional view for illustrating a problem in the driver transistors according to the related art. For convenience, only the contact region 107b at a position of the driver transistor to be formed is shown in the plan view.
As shown in FIG. 9, a substrate resistance 115 of the n-type well 105 is large due to the influence of the substrate potential by impact ionization, so that the substrate potential increases in a position away from the contact region 107b. 
This phenomenon is most remarkable at a position farthest from the contact region 107b, namely, near the middle of the position where the driver transistor is formed.
If the substrate potential is increased due to an inadequate fixation of the substrate potential, an electrical potential at the source region 107s and the substrate potential become electrical potentials with a forward bias direction and a parasitic bipolar transistor inside of the driver transistor begins to act by a current flow, and as a result, a large current flows between the source and drain regions 107s, 107d at a time and the driver transistor causes thermal breakdown.
The thermal breakdown by the parasitic bipolar transistor is a fatal defect for transistors. In this case, there is not only a risk of a breakdown of the device, but also a risk of ignition from an IC (Integrated Circuit) and smoke generation causing a serious accident. Therefore, it is necessary to handle the parasitic bipolar transistor with care.
Several methods to prevent the action of the parasitic bipolar transistor are known. As one of the methods to handle without modifying the transistor structure, a countermeasure by a circuit layout will be explained below.
FIG. 10 shows a schematic plan view and a cross-sectional view for illustrating another example of the driver transistors according to the related art. The cross-sectional view is along the positions signed by Y, Y in the plan view. This will be explained by referring to FIG. 10 as a method to provide a contact region also inside of the driver transistor (for example see Patent Literature 1: JPA H06-275802).
As shown in FIG. 10, the source region 1075 at the middle of where the driver transistor is formed is divided into, for example, two regions. A contact region 107b-1 is added to a region in between the two regions. Herewith, an increase in the substrate potential is prevented also in the middle of where the driver transistor is formed, away from around the edge. As a result, the parasitic bipolar transistor hardly acts.
FIG. 11 shows a schematic plan view and a cross-sectional view for illustrating yet another example of the driver transistors according to the related art. The cross-sectional view is along the positions signed by Z, Z in the plan view. Another method to prevent the action of the parasitic bipolar transistor will be explained below by referring to FIG. 11 as a method to provide a contact region also inside of the source region (for example, see Patent Literature 2: JPA H08-288401).
As shown FIG. 11, a contact region 107b-2 is formed on a part of the source region 107s. A difference with the related art shown in FIG. 10 is that the original source region 107s (a p-type diffuse layer region) abuts on the contact region 107b-2 (an n-type diffuse layer region). In this way, a source whose n-type and p-type diffuse layer regions are formed by abutting each other in the same region is termed “Butting Source”.
The contact region 107b-2 is electrically connected via the contact 113b to a metal wired layer (not shown in the figures) which is electrically connected by the source region 107s and the contact 113s. In summary, the source region 107s and the contact regions 107b, 107b-2 are all set to be the same electrical potential.
The contact 113b is formed on the contact region 107b-2. Moreover, the contact 113b is sometimes formed to cross from above the contact region 107b-2 to above the source region 107s (for example, see Patent Literature 3: JPA 2009-21464, Patent Literature 4: JPA H09-23006 and Patent Literature 5: JPB 2939563).
However, in the related art shown in FIG. 10, the contact region 107b-1 is added to the inside of where the driver transistor is formed, so that there is a problem of increasing the layout area, which means the driver transistor originally occupying a large amount of the layout area requires more of the layout area, which results in an increase in the chip area and the chip price.
In the related art as shown in FIG. 11, the source region 107s composed of the p-type impurity-diffused layer abuts on the contact region 107b-2 composed of the n-type impurity-diffused layer, so that a breakdown voltage of the PN junction is low at this part. Therefore, there is a problem of not being able to set the source region 107s and the contact region 107b-2 at different electrical potentials.
Here, a case was explained in which the contact region 107b-2 is arranged in the source region 107s, but there is the same problem in a case that the contact region 107b-2 is arranged in the drain region 107d. 